Automatic screw machines, CNC lathes and similar bar fed machines have been used for many years to machine small parts in large volumes. These machines provide a reliable, precise and economical way for producing a wide variety of parts, including air and hydraulic fittings, hose fittings, electrical fittings and connectors, carburetor parts, spark plugs, and various transmission and brake parts including shafts, pins, ABS brake cylinder pistons, screws, gears, splines, grease fittings and the like.
In many of these machines, a bar loader supplies bar stock to a plurality of work spindles mounted on a rotary spindle carrier. The carrier is indexed from work station to work station. Tools such as dovetail and circular form tools, drills, taps, reamers, thread rolls and shave tools, located at the different work stations, perform a variety of machining steps on the bar stock. When the machining sequence has been completed, the finished part is severed from the stock, and the stock is advanced so that the cycle may be repeated.
When a finished part is severed from the stock on a multi-spindle automatic screw machine or other bar fed lathe, it is frequently held in a reciprocating burring or pick-off spindle that supports the finished part as it is being cut off from the bar stock. The burring or pick-off spindle is typically driven by a mechanical cam, air actuator or servo-motor so that a collet or chuck in the end of the spindle is advanced over the finished part (on the end the bar stock extending from the work spindle). The collet or chuck is then closed, thus supporting the part while a cut-off tool separates it from the bar stock. The burring or pick-off spindle is coaxial with and driven at the same speed as the work spindle.
Sometimes when the stock is fed out from the work spindle to advance a new length for machining, the stock fails to advance the correct distance due to slippage in the feeding mechanism. Several methods for detecting this type of short feed have been attempted, including sensors positioned in front of the advancing stock which are designed to be activated only when the stock is fed the correct distance. Unfortunately, merely guaranteeing the stock is fed the correct distance will not ensure that the part is the correct length. Short parts can also be the result of the work piece being pushed back into the collet by any one of the tools performing work on the piece. This can result from a poorly adjusted or worn collet, undersized bar stock, or a worn tool which is applying excessive force to the work piece.
Finished parts are generally deposited in a simple parts basket by the machine when completed. If short fed out stock successfully navigates its way through the tooling stations, a too short part will be deposited in the parts basket along with all the good parts. Many customers will reject a complete order if they find even one part in their shipment not made to the print. To avoid this possibility, many screw machine shops are doing 100% inspection in order to sort out the short pieces from the good ones. This is very expensive. Thus, it is vitally important to be able to detect short parts whenever they occur, and take corrective action.